The function mbedtls_mpi_sub_abs first checked that A >= B and then
performed the subtraction, relying on the fact that A >= B to
guarantee that the carry propagation would stop, and not taking
advantage of the fact that the carry when subtracting two numbers can
only be 0 or 1. This made the carry propagation code a little hard to
follow.
Write an ad hoc loop for the carry propagation, checking the size of
the result. This makes termination obvious.
The initial check that A >= B is no longer needed, since the function
now checks that the carry propagation terminates, which is equivalent.
This is a slight performance gain.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
There was some confusion during review about when A->p[n] could be
nonzero. In fact, there is no need to set A->p[n]: only the
intermediate result d might need to extend to n+1 limbs, not the final
result A. So never access A->p[n]. Rework the explanation of the
calculation in a way that should be easier to follow.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
The function mpi_sub_hlp had confusing semantics: although it took a
size parameter, it accessed the limb array d beyond this size, to
propagate the carry. This made the function difficult to understand
and analyze, with a potential buffer overflow if misused (not enough
room to propagate the carry).
Change the function so that it only performs the subtraction within
the specified number of limbs, and returns the carry.
Move the carry propagation out of mpi_sub_hlp and into its caller
mbedtls_mpi_sub_abs. This makes the code of subtraction very slightly
less neat, but not significantly different.
In the one other place where mpi_sub_hlp is used, namely mpi_montmul,
this is a net win because the carry is potentially sensitive data and
the function carefully arranges to not have to propagate it.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
mpi_sub_hlp performs a subtraction A - B, but took parameters in the
order (B, A). Swap the parameters so that they match the usual
mathematical syntax.
This has the additional benefit of putting the output parameter (A)
first, which is the normal convention in this module.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
While pure sh doesn't have a concept of local variables, we can partially
emulate them by unsetting variables before we exit the function, and use the
convention of giving them lowercase names to distinguish from global
variables.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Since dd prints everything on stderr, both normal status update and actual
errors when they occur, redirecting that to /dev/null is a trade-off that's
acceptable in quiet mode (typically used on a developer's machine and the
developer will re-run in non-quiet mode if anything fails without sufficient
detail in the output), but not that much in non-quiet mode.
For example, if our dd invocation fails because the disk in full on a CI
machine, we want the error to be reported at the time we invoke dd, and not
later when a seemingly unrelated test fails due to an incorrect seedfile.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Checking the budget only after the randomization is done means sometimes we
were randomizing first, then noticing we ran out of budget, return, come back
and randomize again before we finally normalize.
While this is fine from a correctness and security perspective, it's a minor
inefficiency, and can also be disconcerting while debugging, so we might as
well avoid it.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
It results in smaller code than using CTR_DRBG (64 bytes smaller on ARMv6-M
with arm-none-eabi-gcc 7.3.1), so let's use this by default when both are
available.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Unless MBEDTLS_ECP_NO_INTERNAL_RNG is defined, it's no longer possible for
f_rng to be NULL at the places that randomize coordinates.
Eliminate the NULL check in this case:
- it makes it clearer to reviewers that randomization always happens (unless
the user opted out at compile time)
- a NULL check in a place where it's easy to prove the value is never NULL
might upset or confuse static analyzers (including humans)
- removing the check saves a bit of code size
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
Currently we draw pseudo-random numbers at the beginning and end of the main
loop. With ECP_RESTARTABLE, it's possible that between those two occasions we
returned from the multiplication function, hence lost our internal DRBG
context that lives in this function's stack frame. This would result in the
same pseudo-random numbers being used for blinding in multiple places. While
it's not immediately clear that this would give rise to an attack, it's also
absolutely not clear that it doesn't. So let's avoid that by using a DRBG
context that lives inside the restart context and persists across
return/resume cycles. That way the RESTARTABLE case uses exactly the
same pseudo-random numbers as the non-restartable case.
Testing and compile-time options:
- The case ECP_RESTARTABLE && !ECP_NO_INTERNAL_RNG is already tested by
component_test_no_use_psa_crypto_full_cmake_asan.
- The case ECP_RESTARTABLE && ECP_NO_INTERNAL_RNG didn't have a pre-existing
test so a component is added.
Testing and runtime options: when ECP_RESTARTABLE is enabled, the test suites
already contain cases where restart happens and cases where it doesn't
(because the operation is short enough or because restart is disabled (NULL
restart context)).
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
While it seems cleaner and more convenient to set it in the top-level
mbedtls_ecp_mul() function, the existence of the restartable option changes
things - when it's enabled the drbg context needs to be saved in the restart
context (more precisely in the restart_mul sub-context), which can only be
done when it's allocated, which is in the curve-specific mul function.
This commit only internal drbg management from mbedtls_ecp_mul() to
ecp_mul_mxz() and ecp_mul_comb(), without modifying behaviour (even internal),
and a future commit will modify the ecp_mul_comb() version to handle restart
properly.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
The case of MBEDTLS_ECP_RESTARTABLE isn't handled correctly yet: in that case
the DRBG instance should persist when resuming the operation. This will be
addressed in the next commit.
When both CTR_DRBG and HMAC_DRBG are available, CTR_DRBG is preferred since
both are suitable but CTR_DRBG tends to be faster and I needed a tie-breaker.
There are currently three possible cases to test:
- NO_INTERNAL_RNG is set -> tested in test_ecp_no_internal_rng
- it's unset and CTR_DRBG is available -> tested in the default config
- it's unset and CTR_DRBG is disabled -> tested in
test_ecp_internal_rng_no_ctr_drbg
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
No effect so far, except on dependency checking, as the feature it's meant to
disable isn't implemented yet (so the descriptions in config.h and the
ChangeLog entry are anticipation for now).
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
* development: (44 commits)
Add test for dependencies on HMAC_DRBG in all.sh
Fix undeclared dep on deterministic ECDSA in test
Document precondition of nonce-generating function in ssl_msg.c
Improve documentation of nonce-generating function in ssl_msg.c
Spell out check for non-zero'ness
Fix debug print of explicit IV
Fix indentation in debug statement in ssl_msg.c
Fix typo in check_config.h
Mention HKDF in TLS 1.3 feature document
Fix typo in header of TLS 1.3 experimental features document
Add dependencies for experimental TLS 1.3 features in check_config.h
Rename TLS 1.3 padding granularity macro
Add documentation on state of upstreaming of TLS 1.3 prototype
Change TLS 1.3 default padding to no padding
Update query_config.c
Fix #endif indicator comment
Add missing configuration guards to SSL record protection helpers
Introduce configuration option for TLS 1.3 padding granularity
Fix copy-pasta in TLS 1.3 record protection unit test names
Fix Changelag PR number and uniformize code when prng fails
...
Let code analyzers know that this is deliberate. For example MSVC
warns about the conversion if it's implicit.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Similarly to the recently-added tests for dependencies on CTR_DRBG:
constrained environments will probably want only one DRBG module, and we
should make sure that tests pass in such a configuration.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
In mpi_montmul, an auxiliary function for modular
exponentiation (mbedtls_mpi_mod_exp) that performs Montgomery
multiplication, the last step is a conditional subtraction to force
the result into the correct range. The current implementation uses a
branch and therefore may leak information about secret data to an
adversary who can observe what branch is taken through a side channel.
Avoid this potential leak by always doing the same subtraction and
doing a contant-trace conditional assignment to set the result.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
Separate out a version of mpi_safe_cond_assign that works on
equal-sized limb arrays, without worrying about allocation sizes or
signs.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This reverts commit 2cc69fffcf.
A check was added in mpi_montmul because clang-analyzer warned about a
possibly null pointer. However this was a false positive. Recent
versions of clang-analyzer no longer emit a warning (3.6 does, 6
doesn't).
Incidentally, the size check was wrong: mpi_montmul needs
T->n >= 2 * (N->n + 1), not just T->n >= N->n + 1.
Given that this is an internal function which is only used from one
public function and in a tightly controlled way, remove both the null
check (which is of low value to begin with) and the size check (which
would be slightly more valuable, but was wrong anyway). This allows
the function not to need to return an error, which makes the source
code a little easier to read and makes the object code a little
smaller.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
The previous version attempted to write the explicit IV from
the destination buffer before it has been written there.
Signed-off-by: Hanno Becker <hanno.becker@arm.com>
* development: (44 commits)
Add test for building without CTR_DRBG
Add usage info of generate_psa_constants script
Fix undeclared deps on CTR_DRBG in programs/fuzz
Fix undeclared deps on MBEDTLS_CTR_DRBG in tests
Finish the documentation of normalize_path
Generate PSA constant names in CMake build dir
Run assemble_changelog.py in all.sh
Wrap line to 79 columns
Some .pem files are openssl output and have tabs and that's ok
.dsw files are Visual Studio stuff
Permit empty files
Normalize line endings
Check only files checked into Git
Exclude binary files from text checks
Regex mechanism for check-specific exemptions
Check all files by default
More accurate variable name
Fix article in documentation
Add output of `python3` version
Add output of make and cmake versions
...
Invasive testing strategy
Create a new header `common.h`.
Introduce a configuration option `MBEDTLS_TEST_HOOKS` for test-specific code, to be used in accordance with the invasive testing strategy.
The list in the pre-push hook was redundant with the list of `check_*`
components in all.sh, and unsurprisingly it was outdated.
Missing components were:
- check_recursion
- check_changelog
- check_test_cases
- check_python_files
- check_generate_test_code
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
The primary purpose is to use it to run all.sh -k -q in the pre-push hook, but
this can be useful in any circumstance where you're not interested in the full
output from each component and just want a short summary of which components
were run (and if any failed).
Note that only stdout from components is suppressed, stderr is preserved so
that errors are reported. This means components should avoid printing to
stderr in normal usage (ie in the absence of errors).
Currently all the `check_*` components obey this convention except:
- check_generate_test_code: unittest prints progress to stderr
- check_test_cases: lots of non-fatal warnings printed to stderr
These components will be fixed in follow-up commits.
Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
